Semiconductor transistors, in particular field-effect controlled switching devices such as a MISFET (Metal Insulator Semiconductor Field Effect Transistor), in the following also referred to as MOSFET (Metal Oxide Semiconductor Field Effect Transistor) and a HEMT (high-electron-mobility Field Effect Transistor) also known as heterostructure FET (HFET) and modulation-doped FET (MODFET) are used in a variety of applications. An HEMT is a transistor with a junction between two materials having different band gaps, such as GaN and AlGaN. In a GaN/AlGaN based HEMT, a two-dimensional electron gas (2DEG) arises at the interface between the AlGaN barrier layer and the GaN buffer layer. In an HEMT, the 2DEG forms the channel of the device instead of a doped region, which forms the channel in a conventional MOSFET device. Similar principles may be utilized to select buffer and barrier layers that form a two-dimensional hole gas (2DHG) as the channel of the device. Without further measures to modify the intrinsic, self-conducting state of the channel, the HEMT device is a normally-on transistor. That is, measures must be taken to prevent the channel region of an HEMT from being in a conductive state in the absence of a positive gate voltage.
Due to the high electron mobility of the two-dimensional carrier gas in the heterojunction configuration, HEMTs offer high conduction and low losses in comparison to many conventional semiconductor transistor configurations. The advantageous conduction characteristics make HEMTs desirable in applications, including, but not limited to use as switches in power supplies and power converters, electric cars, air-conditioners, and in consumer electronics, for example. However, normally-on HEMTs have limited applicability in these applications because these devices must be accompanied by circuitry that can generate the negative voltages necessary to turn the device off. Such circuitry adds cost and complexity to the design. For this reason, it is typically desirable to include features in an HEMT that modify the intrinsic normally-on configuration and provide a normally-off device.
Several designs and corresponding processing techniques have been developed to alter the normally-on aspect of HEMT devices and provide a normally-off device. For example, HEMTs may have a J-FET type structure with a p-type doped gate junction that achieves a threshold voltage (Vth) >0. However, these devices have the disadvantage of limited overdrive capability, due to the opening of the pn-heterojunction. Alternatively, an HEMT having a threshold voltage (Vth) >0 may be achieved by a variety of different MISFET designs with doping in the dielectric insulator between the channel and the gate electrode. However, these MISFET devices suffer from the drawback that the dielectric interface between the gate insulator and the substrate can be unstable. This instability causes significant drifting of the threshold voltage (Vth) drift of the device. That is, the threshold voltage (Vth) of the device changes as the device is biased (either constant or varying) over time. Although trapping behavior is known to contribute to this phenomenon, it has been shown that over the lifetime of a GaN based MISFET structure, the threshold voltage (Vth) drift of the device is not limited by the amount of charge traps, and can theoretically converge with an overdrive bias over a long period of time.
FIGS. 1-3 show the threshold voltage (Vth) drifting behavior of a device, as described by Lagger, et al. in Comprehensive Study of the Complex Dynamics of Forward Bias-Induced Threshold Voltage Drifts in GaN Based MIS-HEMTs by Stress/Recovery Experiments, Electron Devices, IEEE Transactions. vol. 61, no. 4, pp. 1022, 1030, April 2014. The device represented in these figures is a normally-on MISFET formed from 28 nm thick AlGaN/GaN substrate with 25% aluminum content and a 30 nm thick SiO2 passivation layer. Referring to FIG. 1, the change in threshold voltage (ΔVth) is plotted as a function of recovery time for a fixed stress bias (4V) and varying stress time. The stress time is varied between 100 ns, 1 μs . . . to 10 s. Referring to FIG. 2, the change in threshold voltage (ΔVth) is plotted as a function of stress time for a certain recovery time (100 μs) is depicted. The stress bias is varied between 1, 2 . . . to 7V. Referring to FIG. 3, the change in threshold voltage (ΔVth) is plotted as a function of stress bias for a certain time. The stress time is varied between 100 ns, 1 μs . . . to 10 s. The time constants related to this data cannot be directly linked to Schottky-Read-Hall (SRH) behaving defect states described by simple energy levels and cross sections. All known MIS (or MOS) interfaces devices today follow this behavior, and it is particularly pronounced in heterojunction devices.
It is therefore desirable to produce an HEMT that is less susceptible to threshold voltage drifting as described above.